1. Field of the Invention
The invention relates to a toner image transfer apparatus for transferring a toner image formed on an image carrying body, to a recording material such as a sheet, in an image forming apparatus employing an electrophotographic system.
2. Description of the Related Art
In an image forming apparatus such as an image forming apparatus employing an electrophotographic system, a toner image is formed on a photoreceptor as an image carrying body and is transferred to a sheet of plain paper or the like, then the toner of the toner image is fixed on the sheet, for example, by passing the sheet through a heat fixing device, to preserve the toner image on the sheet as a permanent image on the sheet, and lastly the sheet is discharged outside the apparatus.
According to such image forming apparatus, a sheet must be conveyed to a position of image formation in order to form a desired image on the sheet. For example, sheets are received in a paper feed cassette or a tray in large quantity, and a sheet feeding device is provided for feeding sheets one by one from the cassette or the tray to an image forming unit, in particular, a conveying path lead to a transfer position where a toner image formed on a photoreceptor is transferred.
A tip end of a sheet fed through the sheet feeding device is guided to a surface of the photoreceptor in the transfer position. In the transfer position, a transfer apparatus is provided for transfer of a toner image formed on the photoreceptor. The transfer apparatus comprises charging means for charging a back surface of a sheet to be of reverse polarity to the charged polarity of a toner image. The charging means conventionally includes corona dischargers, transfer roller or the like.
The corona dischargers can charge the back surface of a sheet with electricity in non-contact condition, and so do not contaminate the back surface of a sheet. However, problems such as leakage or the like occur due to generation of ozone and supplying of extremely high voltage.
In contrast, in the case where a transfer roller is used, it comes into direct contact with the back surface of a sheet to bring a sheet into pressure contact with a photoreceptor, and so the transfer voltage may be on the order of 1 kV to enable preventing generation of ozone as in corona dischargers, and to decrease problems such as leakage. In the case of transfer rollers, however, toner or the like will not adhere to a transfer roller provided that the roller is brought into pressure contact with the photoreceptor with the sheet therebetween, and when the roller comes into pressure contact with the photoreceptor in a state, in which any sheet is not present, an unnecessary toner remaining on the photoreceptor will adhere to the roller. Therefore, the back surface of a sheet will be contaminated by toner or the like unless the adhered toner is not removed.
Here, in an arrangement, in which a transfer roller or the like is used, a cleaning device is provided for cleaning the surface of the roller. For example, Japanese Unexamined Patent Publication JP-A 51-9840 (1976) proposes means, in place of such cleaning device, in which a transfer voltage for transfer of a toner image is supplied to a transfer roller in a timing that a sheet passes a transfer position, and a cleaning voltage with a polarity reverse to that of the transfer voltage is supplied to the roller in a state, in which a sheet does not pass the transfer position, to shift toner, adhered to the transfer roller, to a side of a photoreceptor for cleaning.
Accordingly, switching control of the transfer voltage and the cleaning voltage causes toner or the like adhered to the transfer roller to be shifted to the side of the photoreceptor, thus making use of a cleaning device on the side of the photoreceptor to perform cleaning, and eliminating the need of any special cleaning device on the transfer roller separately from the cleaning device on the side of the photoreceptor, whereby the apparatus can be made small-sized.
Also, unlike a transfer roller, a transfer conveying belt has been proposed and practiced for conveying a sheet and transferring to the sheet a toner image formed on a photoreceptor. With such transfer conveying belt, a cleaning device or the like is provided for preventing contamination of a sheet due to adherence of toner. For example, Japanese Unexamined Patent Publication JP-A 56-154773 (1981) describes an arrangement, in which a transfer conveying belt is extended to be, charged with reverse polarity to that of a toner image to attract a sheet electrostatically so that the toner image is transferred together with conveying of the sheet.
As described above, with conventional toner image transfer apparatus provided in an image forming apparatus, respective transfer means has advantages and disadvantages, and so are selectively used in accordance with specifications of an associated image forming apparatus.
For example, in the case where a transfer roller is used, it does not offer any problem with respect to transfer of a toner image onto a sheet, but there is the need of providing a peeling-off pawl, which constitutes peeling-off means, placed in contact with a side of a photoreceptor so that a sheet be peeled off the photoreceptor after transfer. To perform peeling-off of a sheet without the provision of such peeling-off pawl, a diameter of the photoreceptor used must be made as small as possible. Therefore, because of the need of rotating the photoreceptor several times (two or more rotations) for the purpose of transferring images contained in a single sheet to the sheet, the number, in which the photoreceptor is shaved off by a cleaning member, is increased, so that it is shortened in life due to film reduction of a photoreceptor layer.
Therefore, when the photoreceptor is made large in diameter, image formation and transfer can be effected with two or less rotations. However, when the photoreceptor is made large in diameter, peeling-off of a sheet problematically remains such that the apparatus cannot be made small-sized due to the need of provision of separate separation mechanism or the like.
Hereupon, a transfer conveying belt is used to attract and convey a sheet, so that, after transfer, the sheet is attracted not by a side of a photoreceptor but by a side of the belt. Therefore, there is no need of providing a separate separation mechanism or the like, and it is possible to cope with high speed processing without being affected by a diameter of the photoreceptor.
In the case of using the transfer conveying belt, however, it is inevitably necessary to provide a cleaning device separate from a cleaning device on the side of the photoreceptor, so that it cannot be desired to further make the image forming apparatus small-sized. Besides, the transfer conveying belt is formed to be exceedingly lengthy, which correspondingly makes the apparatus large.
As described above, a system, in which a transfer roller is brought into contact with a back surface of a sheet to transfer a toner image thereto, has advantages and disadvantages, so that it is not possible to perform high speed processing, make an apparatus small-sized, and suppress an increase in cost. For example, in the case of using a transfer roller system, a cleaning device is dispensed with by causing toner or the like adhered to the transfer roller to be transferred to a photoreceptor, but it is necessary to separately provide a peeling-off mechanism at all means. The use of a transfer belt system can omit a peeling-off mechanism, but the transfer belt is made lengthy, which does not make miniaturization. Further, with an arrangement, in which toner or the like adhered to the transfer belt is transferred to the photoreceptor with a view to cleaning, cleaning takes time, and so the apparatus cannot cope with high speed running.